Recently, a hybrid vehicle mounted with an engine, an electric motor, and a generator, has been used on various occasions, as has an electric vehicle mounted with an electric motor and a generator. The hybrid vehicle is driven according to its travelling state. For example, the hybrid vehicle may be driven by a combination of output of the engine and the electric motor, or by using part of the output of the engine to operate the generator and charge a battery, while combining the remaining output of the engine and the output of the electric motor to drive the vehicle. Alternatively, the output of the engine may be used to drive the generator, and the generated power is used to operate the electric motor to drive the vehicle. In many cases, such hybrid vehicles or electric vehicles use a boosting converter to boost a direct current (DC) low voltage of a battery to a DC high voltage that is supplied to inverters where the power is received and supplied between the electric motor and the generator. The inverters operate the electric motor by converting the DC power into a three-phase alternating (AC) power for operating the electric motor, or convert the three-phase AC power generated by the generator into DC power.
The boosting converter is configured to boost a DC low voltage of a battery to output a DC high voltage by turning on and off switching elements and using energy stored in a reactor, in which a boosting loss is generated by turning on and off the switching elements. The boosting loss increases as output power and a boosting ratio (ratio of the DC high voltage to the DC low voltage) of the boosting converter increase, and vice versa. However, even in a no-load state where the output power of the boosting converter is zero, the boosting loss (switching loss) does not come to zero so long as the switching element is carrying out the on-off operation.
In the hybrid vehicle or the electric vehicle, when the vehicle drives in a travelling state where the power generated by the generator is balanced with the power consumed by the electric motor, the vehicle can continue traveling while maintaining the DC high voltage of the inverters. This is because the electric motor can be operated in this state by using only the power generated by the generator without using the DC high voltage obtained by boosting the DC low voltage of the battery. Since the boosting converter has no load in this case, it may be possible to suspend the operation of the boosting converter to decrease the boosting loss (switching loss) in order to improve efficiency of the vehicle system. However, it is difficult to maintain a perfect balance of the power consumed by the electric motor and the power generated by the generator. For example, if the power consumed by the electric motor is somewhat larger than the power generated by the generator, the DC high voltage of the inverters may gradually decrease upon suspension of the boosting converter. In view of this, there has been proposed a method in which, when the power generated by the electric motor is balanced with the power consumed by the generator, the operation of the boosting converter is suspended and the output torque of the electric motor is corrected in order to decrease the deviation between the DC high voltage of the inverters and a target voltage, with an output power of the generator being fixed. Accordingly, the power generated by the generator is balanced with the power consumed by the electric motor, and the DC high voltage of the inverters can be maintained.